Priority is claimed from Japanese Patent Application No. 2017-006036 filed on Jan. 17, 2017. The content of this application is incorporated herein by reference in its entirety.
The present disclosure relates to a power amplifier module. A mobile communication device such as a cellular phone is provided with a power amplifier for amplifying a transmit signal. For example, Japanese Unexamined Patent Application Publication No. 2012-54874 discloses a radio-frequency amplifier including, to meet requirements for high output power, a power divider circuit that divides an input signal into signals, a set of amplifier elements that amplify the respective signals that are divided, and a power combining circuit that combines the amplified signals. The radio-frequency amplifier includes an isolation resistor that electrically connects output terminals of the set of amplifier elements to each other, and a passive element connected in parallel to the isolation resistor to increase impedance. Thus, if a signal from one path enters the other path in an unbalanced mode, the isolation resistor absorbs power, which ensures isolation between the amplifier elements.
There is a demand for improved performance of power amplifiers in accordance with a change in the communication standard to be applied. In communication standards such as Long Term Evolution-Advanced (LTE-Advanced), carrier aggregation that enables simultaneous transmission of a plurality of transmit signals of different frequency bands is used. Power amplifiers with improved output power and improved output linearity are demanded accordingly.
Typically, the performance of an amplifier element is maximized when the impedance on the load side as seen from the amplifier element is approximately a real number. In this regard, in the configuration disclosed in Japanese Unexamined Patent Application Publication No. 2012-54874, a single passive element performs impedance matching between the amplifier elements and the load side of the radio-frequency amplifier, which results in a substantially maximum imaginary part of the impedance of the amplifier elements on the load side. Accordingly, depending on the configuration disclosed in Japanese Unexamined Patent Application Publication No. 2012-54874, there is a limitation on improvements of the performance of the amplifier elements, and it is difficult to improve linearity.